There are various theories on the causes and development of Alzheimer's disease. But, it is very clear that it involves a build-up of beta amyloid with subsequent deterioration of cognitive ability.
The beta amyloid is formed from microglia, which are the macrophages of the brain. Microglia have several important functions in the brain. However, the microglia can become hyperactive and, as a result, cause the production and build-up of beta amyloid in the brain, which subsequently causes cognitive deterioration. Generally, most of the beta amyloid is produced prior to any clinical detection of even mild cognitive impairment. By the time of diagnosis, up the 50% of the brain mass may have been lost. Thus, any treatment of Alzheimer's should occur prior to the onset of dementia and, preferably, much earlier.
Further, as with most neurodegenerative diseases, there is also a deterioration of the blood brain barrier.